Configuration #2 - Equatorial

This configuration was a development of Configuration #1 (HI observations) - but was never used operationally as the benefit from sidereal motion tracking was considered not sufficient enough to warrant the extra complexity. This is brought about by the limited view of the sky here at HawkRAO due to extensive tree coverage at the observatory site.

True Equatorial Mount

The following are details of converting the 'polar mount' of the satellite dish into a true equatorial mount.

TVRO Polar Mount

The mount supplied with the 3 m TVRO dish is called a 'polar mount' - a name, presumably, derived from the need to align the rotation axis of the mount to either the north or south celestial poles (depending on the location of the dish north or south of the equator respectively). The mount is designed to allow the dish to be moved (by the actuator) around the axis to point the antenna at various satellites placed in a circle above the equator in the Clarke Belt. The rough diagram below shows the original intent of the 'polar mount'.

NOTE: there is a typo in both the following diagrams: 34.5° should be 33.5°.

The normal TVRO satellite configuration involves aligning the rotation axis with the north or south celestial pole. If you live on the equator the rotation axis is simply horizontal (elevation = 0°), azimuth due north/south and that would be the end of it as the satellites are overhead in an arc and the dish pointing direction is at right angles to the rotation axis. However, if you live north or south of the equator two adjustments to that simple case are needed.

First, the rotation axis would need to be elevated by the same angle as the local latitude and the azimuth, once again, north for the northern hemisphere, and south for the southern hemisphere.

Second, an offset adjustment needs to be made because of the 'parallax error' at different receiving latitudes due to the satellites' distance (35,790km) being a relatively small multiple of the Earth's radius. If the satellites were millions of kilometres away then no offset would be required. For receiving locations south of the equator the offset kicks the dish north to point at the satellites, while for south locations the kick is to the south. See the green detail in the above diagram. This offset is provided by the threaded rod mounting point - 0° for equatorial latitudes and a maximum of 10° at polar latitudes.

Equatorial Mount Pointing to Vela

At the latitude of HawkRAO (33.5°S), the required offset for the Vela Pulsar is in the opposite direction to the offset for TVRO satellites. The required offset magnitude is 45.18° as shown in red in the " 'Polar Mount' Configurations" diagram above. The basic arrangement for a true equatorial mount is shown below…

The dish would need to be jacked up around pivot point 'A' by the grey bar ('B-C') by 45°, the declination of Vela. The offset adjustment threaded rod at point 'A' is normally used when pointing at satellites to provide an offset to the dish needed compensate for the relative closeness of the satellites - i.e., they are not exactly at declination 0°. This adjustment can conveniently be used to allow trimming of the declination pointing of the dish to account for small deviations of the antenna beam from the measured physical orientation of the dish.

Taking some measurements and drawing a rather busy diagram below allows the calculation of the length of the 'jack up' extension bar.

The threaded 'TVRO offset' rod can now be used to trim the declination around 45°S with a range of about ±5° to compensate for any smallish skew of the beamwidth which may occur. In the diagram the threaded rod has been set to the middle of its range. The calculated distance between the mounting holes for the 'jack up' extension bar (attached to points 'B' and 'C') is 291 mm.

NOTE: a typo was made in the drawings. An angle of 34.5° was marked - whereas it should have been 33.5°.

This 'jacked-up' mounting arrangement was, to use a technical term, a bit 'wobbly'. To stabilise the mounting on the extension bar two turnbuckles plus D-buckles were used to triangulate the mounting ring and the base of the extension bar as shown below.

Equatorial, or polar mounts, are not common amongst radio telescopes - especially large ones (the Green Bank 42m is one of the few). Probably the largest number of examples would be found in small amateur radio telescopes which are converted TVRO polar mounts (like the one being described here) or, for very small antennas, converted optical astronomy telescope mounts.

The next task is attaching a linear actuator (TV satellite actuators are fine as they have a pulse output to keep track of the movement of the actuator arm. The actuator is attached to the standard mounting point.

The upper mounting point attachment also had to be modified due to the dish being jacked up at 45° from its usual TVRO position...


Further development was terminated as the extra complexity required to add declination pointing to sidereal tracking was considered too complex for the benefit received.